Fieldbus networks provide communications for process plant equipment such as pressure, temperature and flow transmitters and valve actuators. Fieldbus networks typically extend over wide areas of a process plant environment. Control networks perform higher level process control functions and are typically located in one or more control rooms that are located away from the process plant environment.
The fieldbus network carries real time process data from the transmitters and actuators. There is a desire to transfer or “bridge” the real time process data from the fieldbus network to the control network so that the control network can quickly access information on conditions in the process plant.
If the fieldbus network and the control network are “bridged” or connected by conventional connections, however, the results are not satisfactory. The fieldbus network is typically grounded to a point in the process plant environment (process ground), and the control network in grounded to a point in the control room environment (control network ground). When the fieldbus network and the control network are bridged, an undesired ground loop is formed. Differences in potential between the process ground and the control network ground cause noise currents to flow over network ground conductors. The noise can result in poor performance of both the fieldbus network and the control network.
In addition, the data on the fieldbus network is formatted according to a fieldbus data format or protocol which is typically not compatible with data formatting in the control network. Data format conversion is needed in order to transfer data from the fieldbus network to the control network. Data format conversion, however, requires power and supplying the power needed to convert the data can result in drawing an excessive amount of power from the fieldbus network, or a further undesired ground loop.
A system is desired that transfers data from a fieldbus network to a control network without introducing undesired ground loops along either a data format conversion path through the system or a power supply path through the system.